The present disclosure generally relates to methods for assaying polymers, and, more specifically, to methods for determining a molecular characteristic of a polymer using an integrated computational element.
The physical properties of various polymers can often be a determining factor as to whether a particular polymer can be used successfully in a given application. For example, polymer physical properties such as molecular weight and degree of crosslinking can dictate the viscosity of a fluid phase in which the polymer is disposed. Due to their molecular complexity and frequent batch-to-batch variability, it can sometimes be desirable to assay the physical properties of a polymer before its use in an application in order to ensure that the polymer is capable of performing as intended. The physical properties of a polymer may also be referred to herein as a “molecular characteristic” of the polymer. As used herein, the term “molecular characteristic” will refer to an observable physical property of a plurality of polymer molecules, where the physical property is determined by the polymer's molecular structure.
In the case of small molecules, which have well-defined molecular structures of finite size, physical property measurements can be carried out by various techniques, including both spectroscopic and laboratory analyses, in order to determine a substance's suitability for a particular use. Such analyses of small molecules can often be carried out rapidly.
Polymers, in contrast, are much more difficult to analyze due to their high and variable molecular weights and much more complicated molecular structures. Even though many polymers interact extensively with electromagnetic radiation, their complex molecular structures can make it very difficult to extract meaningful structural information from a conventional spectroscopic assay. Physical properties of polymers are most often assayed using non-spectroscopic laboratory analyses, many of which are fairly time consuming and only of limited accuracy. Although time consuming, these non-spectroscopic analytical techniques can be satisfactory in many cases, at least for purposes of characterizing a pristine polymer. However, when it is desired to know how a polymer is performing in an ongoing application (i.e., in the field), for example, these analytical techniques can sometimes be unsatisfactory. In many instances, physical property assays for polymers are performed with the polymer dispersed in a fluid phase, and colligative property measurements on the fluid phase may be used to determine a physical property of the polymer. For example, colligative property measurements may be used to determine a polymer's molecular weight. However, colligative property measurements usually require careful and time-consuming standardization protocols and the use of a high purity fluid phase. Hence, for field-derived polymer samples, particularly field-derived polymer samples obtained in a fluid phase, standard polymer analyses based upon colligative property measurements may be too slow or inaccurate to provide meaningful physical property information, or may simply not be possible to perform.
In addition to assaying the physical properties of a polymer during or following an application, it can also be desirable to measure the physical properties of a polymer during or following its synthesis. For example, it can be desirable to determine if a particular set of reaction conditions is producing or has produced a polymer having a desired set of physical properties (e.g., molecular weight, degree of crosslinking, degree of branching, crystallinity, and the like). As one of ordinary skill in the art will recognize, typical polymer reaction mixtures may be very complex and non-amenable to the common polymer characterization techniques discussed above. On the supply side, there can often be considerable batch-to-batch variability of polymers, even those obtained from the same manufacturer. Thus, for quality control purposes, it can be highly desirable to determine a polymer's physical properties before deployment in a given application.
Although certainly not limited to this field of use, polymers are often employed extensively in the oilfield services industry. As discussed previously, the physical properties of a polymer can heavily impact the polymer's performance in a given application, and oilfield applications are no exception. The issues encountered in the oilfield services industry in regard to the physical properties of polymers are considered to be representative of those encountered in other fields. Illustrative uses of polymers in oilfield applications can include, for example, as a viscosifying or gelling agent, a friction reducer, a sealant composition, a diverting agent, a scale inhibitor, a relative permeability modifier, or the like, in various treatment fluids. Treatment fluids and treatment operations are described in more detail below. In addition to their use in treatment fluids, polymers may comprise various parts of downhole tools.
A treatment operation or tool employing a polymer having an incorrect physical property may fail due to the polymer not being capable of functioning as intended in a subterranean formation. For example, a polymer having an incorrect physical property may not convey satisfactory properties to a treatment fluid in which it is disposed, and the treatment fluid may then not perform as intended during a treatment operation. As an illustrative example, the degree of crosslinking may dictate the effective performance lifetime of a polymer and/or lead to premature fluid breaking. As another illustrative example, a treatment fluid containing a polymer with an incorrect molecular weight may not have a suitable viscosity (a function of the polymer's molecular weight and/or degree of crosslinking), which can impact the treatment fluid's ability to carry proppant particulates, divert a fluid in a subterranean formation, and the like. Hence, it can be desirable to assay for the physical properties of a polymer before or while forming a treatment fluid therefrom, including “on-the-fly.”
The problem of assaying for the physical properties of polymers in the oilfield is even more complicated during and following a treatment operation. Before performing a treatment operation, some delay in analyzing the physical properties of a polymer or a treatment fluid formed therefrom is at least tolerable, although not preferable. However, for assaying a polymer or a treatment fluid during or after a treatment operation, the issues are much more complicated and may be difficult or impossible to overcome by conventional polymer analyses. Downhole or post-production analysis of a polymer may be desirable to determine if a satisfactory break has occurred, for example. However, the complex nature of produced fluids and formation fluids can make such polymer analyses difficult or impossible to perform by conventional techniques. Even to the extent that a produced fluid can be sampled and further characterized, significant analytical delays may lead to analyses that are not representative of the polymer's physical properties while downhole. Downhole analyses can also be particularly difficult to perform due to the harsh nature of the subterranean environment.